FIGS. 6 and 7 show a prior art spiral gasket. In the figures, reference numeral 1 represents a hoop member consisting of a metal thin plate (about 0.13 mm.about.0.15 mm thick) having an undulated cross-section and 2 indicates a filler member made mainly of inorganic fiber, etc.
In order to fabricate a spiral gasket constructed as described above, at first only the hoop member 1 is wound around a take-up piece (not indicated in the figures) as much as two or three turns (idle winding) and then a welded fixed portion 3 is formed by spot welding at a predetermined position in this idle winding part. Thereafter a filler member 2 is inserted between different turns and the hoop member and the filler member are wound, superposed on each other. At the end of the winding, again only the hoop member is wound and different turns of the hoop member are secured to each other by spot welding.
The welded fixed portion 3 of the hoop member on the inner side is formed at a position distant from the beginning of the winding by about a quarter turn for a reason of the fabrication. Since a part 1a of the hoop member corresponding to the quarter turn remains unattached and a burr at the extremity thereof is dangerous at handling it, the part 1a of the hoop member is cut at the proximity of the welded fixed portion 3 to be removed for obtaining a final product. In FIG. 6, 1 corresponds to about a quarter turn.
FIGS. 8 and 9 show a state where the final product of the spiral gasket, thus obtained, is put between flanges formed at extremities of pipes in a pipe connecting portion of a flue gas outflow duct on a vehicle to be used, in which A indicates the spiral gasket thus constructed; 4 and 5 are flue gas outflow pipes; 6 and 7 are flanges; 8 and 9 are a bolt and a nut, respectively, for fastening them together.
When the spiral gasket is mounted on a flue gas outflow pipe, in the case where the flue gas outflow pipe is in a horizontal state with respect to the floor surface, as indicated in FIG. 8, the gasket mounted thereon is supported stably at a predetermined position. However, in the case where the flue gas outflow pipe is in an inclined state with respect to the floor surface, the gasket mounted thereon is apt to fall off. In this connection, when a spiral gasket having an inner diameter of 50.0 mm and an outer diameter of 65.0 mm is used for a flue gas outflow pipe having an outer diameter of 48.6.+-.0.3 mm, there is a gap of 1.1 to 1.7 mm therebetween and thus the gasket falls off easily from the flue gas outflow pipe. For this reason, the bolt and the nut should be fastened in a state where the gasket is pressed down so as not to fall off from the flue gas outflow pipe and therefore efficiency of work for mounting the gasket is extremely low.